1. Field of the Invention
This invention relates to a circuit arrangement for the frequency-dependent analysis of received signals which lie in a wide overall frequency band utilizing a filter bank which cooperates with an evaluation circuit, and more particularly to such an arrangement in which the filter bank has frequency-dependent band pass filters and comprises an evaluation frequency range which exhibits only a few small bandwidth in relation to the overall frequency band, and wherein the received signals are processed by a filter circuit having adjoining pass ranges so that the receiving signals are modulated into the evaluation frequency range by means of a number of frequency modulators, and wherein the individual heterodyning frequency values are added.
2. Description of the Prior Art
A radar apparatus is known from the publication "Proceedings of the IRE", Vol. 49, 1961, No. 9, pp. 1417-1423, in which a frequency analysis is undertaken in order to determine the Doppler frequency. The appertaining circuit arrangement operates with a first filter bank, roughly subdividing the practicable frequency range and consisting of a number of band pass filters connected in parallel. On the basis of the rough information thus obtained, that oscillator is then selected from a number of oscillators, by means of a corresponding selection circuit, which permits bringing the received signal into the pass range of a second filter bank which comprises a number of very narrow-banded band pass filters. As a certain time passes for the first frequency analysis in the first filter bank, roughly subdivided, the received signal, in particular the reflected echo pulse, is stored for such a time until the necessary preliminary decisions are made, i.e. the selection of the respective low frequency oscillator. Not until then is a received signal passed on to the finely subdivided second filter bank for a further evaluation.
This known circuit arrangement has the disadvantage that its construction is very expensive. Moreover, it is then no longer suited if a very wide frequency band is to be monitored or is to be examined for the presence of signals.
A difficulty generally occurring in known circuit arrangements which serve for a frequency-dependent analysis of received signals resides in the fact that signals which fall into the transition range, e.g. a "gray range", between two successive pass ranges are either lost in the evaluation due to diminished sensitivity, or provide indication errors. This cannot be completely prevented, no matter how steep the filter response flanks, whereby the disadvantage exists that such filters are complicated and expensive.